Inkjet printing of δ-MnO₂ nanosheets for flexible solid-state micro-supercapacitor

Inkjet printing is considered as a promising technique for flexible electronics fabrication owing to its simple, versatile, environmental-friendly and low-cost features. The key to inkjet printing is ink formulation. In this work a highly concentrated ink containing two-dimensional δ-MnO₂ nanosheets with an average lateral size of 89 nm and around 1 nm thickness was used. By engineering the formulation of the δ-MnO₂ ink, it could be inkjet printed on O₂ plasma treated glass and polyimide film substrates to form δ-MnO₂ patterns without undesired “coffee-ring” effect. As a proof-of-concept application, all-solid-state symmetrical micro-supercapacitors (MSCs) based on δ-MnO₂ nanosheet ink were fabricated. The fabricated MSCs showed excellent mechanical flexibility and good cycling stability with a capacitance retention of 88% after 3600 charge-discharge cycles. The MSCs attained the highest volumetric capacitance of 2.4 F cm⁻³, and an energy density of 1.8·10⁻⁴ Wh cm⁻³ at a power density of 0.018 W cm⁻³, which is comparable with other similar devices and show great potential as energy storage units for low-cost flexible and wearable electronics applications.